The nature of DNA structures which stimulate genetic recombination (recombinogenic structures), of the molecular processes which involve these structures, and of the proteins which mediate these processes, are to be characterized. Most experiments employ DNA lambda duplication phages and the recombination and DNA repair systems of Escherichia coli. A biological (transfection) assay for recombination and DNA repair, and a biochemical (restriction-electrophoresis) assay for recombination will be used. The identity of the product of the E. coli arl gene, the structure of the single-strand-like regions in DNA of phage grown on arl-mutants, and the mechanism by which these "Arl-" lesions stimulate recombination are to be elucidated. The recombinogenic photoproducts in UV-irradiated DNA will be determined, and the recombinogenic structures resulting from repair of UV-damaged DNA and uracil-containing DNA characterized. Cell-free systems, based on repair-recombination of UV-DNA, will be used to determine the biochemical roles of DNA gyrase and the Rec A protein. A novel technique for obtaining new classes of E. coli "hyper-rec" mutants is proposed. Biochemical analysis of the phage lambda "Red" recombination system will be irradiated.